I. Field
The present invention relates generally to communication, and more specifically to techniques for performing cell reselection in a wireless communication system.
II. Background
In a Global System for Mobile Communications (GSM) system, a terminal that is just powered on or has just lost coverage searches for suitable cells from which the terminal can receive communication service. A “cell” can refer to a base station in the system and/or the coverage area of the base station, depending on the context in which the term is used. A “suitable” cell is one that the terminal can receive service on. GSM defines a set of criteria that a cell must meet in order to be deemed a suitable cell. If a suitable cell is found, then the terminal performs registration with the cell, if necessary. The terminal then “camps” on the cell if the terminal is in an idle mode and not actively communicating with the cell. While camped on the cell, the terminal performs tasks as specified by GSM so that the terminal can (1) receive system information from the cell, (2) receive paging messages from the cell (e.g., alerting the terminal to incoming calls), and (3) initiate call setup for outgoing calls or other actions. The cell on which the terminal is camped is referred to as the “serving” cell.
While camped on the cell, the terminal periodically checks to see if there is a better cell that the terminal can camp on and receive service (e.g., another cell with a higher received signal level). If such a cell exists, then the terminal selects this cell as the new serving cell via a process commonly referred to as “cell reselection.” The terminal may also be required to immediately perform cell reselection to another cell under certain scenarios. For example, the terminal is required to immediately perform cell reselection if the current serving cell becomes barred, if the terminal cannot receive the signal from the current serving cell because the channel condition has degraded, and so on. In any case, the terminal performs cell reselection while in the idle mode so that it can monitor the system for incoming paging messages and initiate a call even if the channel condition changes (e.g., if the terminal moves to a new location).
For cell reselection where the initial reselection fails or there is no information on neighbor cells, the terminal may need to obtain received signal strength measurements and need to gather pertinent system information for new cells, both of which typically require a long period of time to perform. During the time that the terminal is performing these tasks, it is not able to receive service from the system and would also miss any paging messages sent to it, both of which are highly undesirable.
There is therefore a need in the art for techniques to expediently gather information about neighbor cells during cell reselection in order to reduce both down time and the likelihood of missing paging messages.